Despite great progress in the treatment of pediatric cancers, mortality among children with high-risk neuroblastoma remains high, with nearly 60% of the patients developing recurrent disease that is incurable with existing therapies. Traditional cancer treatment use drugs that target the neoplastic cells. However, it is now established that the non-neoplastic components of the tumor (i.e., the tumor stroma/microenvironment) can either promote or inhibit tumor growth depending upon its cellular composition and the organization of the extracellular matrix. Matricellular proteins are natural regulators of the interactions between cancer cells and their microenvironment. We and others have shown that the matricellular protein Secreted Protein Acidic and Rich in Cysteine (SPARC) regulates the assembly and composition of the extracellular matrix, blocks angiogenesis, and inhibits the growth of neuroblastoma and other neoplastic tumors. We have conducted structure-functional studies by testing the anti-angiogenic activity of SPARC constructs with deletions in the 3 major domains, and a number of SPARC peptides. We have further demonstrated that peptide FS-E (22 amino acids), which corresponds to an EGF module of a follistatin domain, has anti-tumor activity. Most recently, we have designed a smaller SPARC peptide (11 amino acids), FSEC, corresponding to the C-terminal part of FS- E, and have demonstrated that it also has potent anti-angiogenic activity and inhibits the growth of neuroblastoma in preclinical models. Further, our preliminary studies show that FSEC inhibits the growth of pancreatic, lung, and breast cancer xenografts, indicating that FSEC may have therapeutic activity in a broad range of tumor types. The specific aim of this study is to elucidate the mechanisms by which peptide FSEC inhibits angiogenesis and neuroblastoma tumor growth by identifying FSEC-binding partners through: 1) cross-linking FSEC engineered with a diazirine motif with human neuroblastoma tumor tissue proteins; 2) purifying binding partners by affinity chromatography, and 3) identifying protein partners with mass-spectrometry. We will use a systemic proteomic approach to isolate the proteins that interact with FSEC. To engineer FSEC probes, we will use new technology that incorporates a highly reactive diazirine group into synthesized peptide probes. These probes will be incubated with human neuroblastoma tumor tissue, and cross-linking will be performed under UV-irradiation. Binding partners will be purified by affinity chromatography and identified by mass-spectrometry. Identification of the proteins that interact with FSEC will lead to a better understanding of how FSEC inhibits angiogenesis and tumor growth, and will provide insight for the development of novel therapeutics that target non-neoplastic cells in the tumor by creating a non- permissive microenvironment. The long-term goal of our research is to improve the outcome of children with high-risk neuroblastoma by developing effective therapeutic strategies that target the microenvironment.